Traditional line voltage dimmers use phase angle control to control the amount of power delivered to a load. The line voltage dimmer chops the alternating current line voltage period and delivers power to the load only for a fraction of the period. The longer the dimmer conducts the current, the larger is the amount of power supplied to the load. Different methods can be used to deliver power to the load. One method uses standard phase control, where the load is connected to the line voltage at a certain point or a certain angle in the AC period and remains connected until the next zero pass. In this case, the current doesn't flow to the load until the desired AC phase angle is reached, as illustrated in FIG. 1a. This method is sometimes referred as a “leading edge dimmer”. Another method uses reverse phase control. The load is connected to the line voltage from the beginning of the AC period (zero angle) and is switched off at a certain point in the AC period. The current therefore flows from the zero angle until the desired angle is reached and the current is then switched off, as illustrated in FIG. 1b. This method is sometimes referred as a “trailing edge dimmer”.
Conventional dimmers are built to control linear loads. Linear loads are loads that draw sinusoidal current corresponding to the applied sinusoidal voltage as shown on FIG. 2a. Conventional dimmers are built to control linear loads with either no phase shift or a small phase shift between the applied voltage and the load current. FIG. 2a depicts a linear load without phase shift. Such loads may be, for example, incandescent lamps or halogen lamps, even if they are powered through low voltage magnetic transformers.
LED lamps, CFL lamps, electronic low voltage transformers and similar devices are examples of non-linear loads, where the current does not correspond to the sinusoidal input voltage, as shown on FIG. 2b. 
The difficulty in dimming non-linear loads is illustrated in FIG. 3. First, looking at how a linear-load is dimmed, as illustrated in FIG. 3a, notice that the load current corresponds nicely to the dimmer output voltage. When the dimmer “clips” the voltage, load current corresponds to the clipped voltage. When the voltage is reduced, the current is reduced accordingly. The power output is indicated in terms of % to indicate an approximation of the output for different dimming levels. The indicated power levels are approximations to show the principle of operation and are not precise values.
Dimming with conventional dimmers is possible because the current is predictable and it corresponds to the chopped voltage. If the AC period is chopped at the predetermined levels, the power delivered to the load is also correspondingly predetermined.
FIG. 3b shows the dimming of a non-linear load. With a non-linear load, the current does not correspond to the voltage in a predetermined and predictable way. In FIG. 3b, if the input voltage is reduced by clipping, the current does not change for some time, then it quickly drops to zero and again the current does not change in correspondence with the following dimming steps.
The output characteristic for a linear load is shown in FIG. 4a, the output characteristics for a non-linear load shown in FIG. 4b. The output characteristic for the non-linear load clearly shows that setting the output to a desired level for such a load would be quite difficult, since most of the input variation does not produce any output change. Only a very limited input range actually results in a change in output. Consequently, very small changes in input in this range make very dramatical output changes, making this control overly sensitive and completely impractical. Even worse, each non-linear load can have a completely different characteristic, and the current spike can even fall outside the control range of a conventional dimmer. This example is shown in FIG. 5, where changing the input across the whole range does not produce any significant change in output.
For some non linear loads, the current characteristic changes depending on the applied voltage. If a chopped voltage is applied to such load, the current spike shape and position can unpredictably change, depending on the amount of the chopped voltage applied. This makes the load current even more unpredictable and harder to control with conventional dimmers.
Consequently, attempting to control the power output for most non-linear loads, using a conventional dimmer is difficult to impossible.
Dimmers suffering from the above described problems include the conventional standard phase control dimmers described in U.S. Pat. Nos. 3,684,919 or 3,397,344, and the reverse phase control dimmers described in U.S. Pat. Nos. 4,528,494 or 5,038,081.
One approach to this problem is to modify the non-linear load itself, for use with a conventional dimmer. This generally involves designing the non-linear load to display load characteristics that mimic linear loads. Special circuits or circuit designs need to be incorporated into the non-linear load for this to work, increasing the cost, complexity and size of the load. Examples of such modified loads include dimmable electronic low voltage transformers, dimmable LED's, dimmable CFL's, etc., U.S. Pat. No. 6,172,466 being an example.
While dimming of such devices with conventional dimmers is possible, including special circuits inside the non-linear loads makes them more complex and expensive. This method does not change the ability of the dimmer to regulate power of the non-linear load, but rather attempts to make non-linear load linear.
Another approach is to incorporate a dedicated power controller with the non-linear load. The controller can be built into the load or be a separate unit wired to the load, so that the load can be accessed and controlled via dedicated wires, or via signals superimposed on power lines or another similar method. This solution is also expensive since special circuits and in some cases special wiring is needed. Examples of such designs are described in U.S. Pat. No. 7,358,679,
In U.S. Pat. Nos. 4,350,935, 4,527,099 and 4,728,866, various methods of regulating power of inductive loads (such as HID and fluorescent lamps with magnetic ballasts) are described which utilize a modified phase control method. This method is useful for linear loads with large phase shift between current and voltage and would work on linear inductive loads, or even possibly on resistive and capacitive linear loads, but would not be useful for non-linear loads since the method assumes the load current will follow the chopped AC voltage in a predictable way, which is not the case with non-linear loads.
Another approach could be to reduce the AC voltage while retaining the sinusoidal form via some sort of PWM, as described for example in U.S. Pat. No. 5,691,628. This method may be able to control power of most linear and non-linear loads, but the component count and complexity of such a circuit makes it very expensive to implement. Furthermore, the higher switching frequencies used in such circuits produce more switching loses, making it less efficient.